Osmotic swelling of heart mitochondria in acetate and chloride salts: Evidence for two pathways for cation uptake

Abstract

Swelling of nonenergized heart mitochondria suspended in acetate salts appears to depend on the activity of an endogenous cation/H + exchanger. Passive swelling in acetate shows a characteristic cation selectivity sequence of Na + >Li + >K +, Rb +, Cs +, or tetramethylammonium, a sharp optimum at pH 7.2–7.3, activation by Ca 2+, and loss of activity on aging which can be related to loss of endogenous K +. The reaction is nearly insensitive to either addition of exogenous Mg 2+ or removal of membrane Mg 2+ with EDTA. Each of these characteristics of passive swelling in acetate salts is duplicated in chloride media when tripropyltin is added to induce Cl −/OH − exchange. In contrast to nonenergized mitochondria, swelling of respiring mitochondria has been postulated to depend on electrophoretic uptake of cations in response to an interior negative membrane potential. Respiration-dependent swelling in acetate shows an indistinct cation selectivity sequence with Li + and Na + supporting higher rates of swelling at higher efficiency than K +, Rb +, and Cs +. The high rates of respiration-dependent swelling in Li + and Na + are inhibited by low levels of exogenous Mg 2+ ( K i of 5–10 μ m), but a significant swelling with almost no cation selectivity persists in the presences of 2 m m Mg 2+. Removal of membrane Mg 2+ by addition of EDTA strongly activates the rate of respiration-dependent swelling and converts a sigmoid dependency of swelling rate on Li + concentration to a hyperbolic one with a Km of about 14 m m Li +. The cation selectivity and Mg 2+ dependence of the reaction induced in chloride salts by tripropyltin are identical to these properties in acetate. Energy-dependent swelling in acetate shows optimum activity at pH 6.5 which appears related to the availability of free acetic acid, since the corresponding reaction induced in chloride shows a broad optimum at about pH 7.5. These studies support the concept that monovalent cations enter nonenergized mitochondria by electroneutral exchange with protons but penetrate respiring mitochondria by electrophoretic movement through one or more uniport pathways. They further suggest that both a Mg 2+-sensitive uniport with high activity for Na + and Li + and a Mg 2+-insensitive pathway with little cation discrimination are available in the membrane.